Marine seismic exploration is normally conducted by firing a seismic source towed close to the sea surface by a vessel. The seismic energy is propagating down through the earth and parts of the transmitted energy will return to the surface after being reflected and/or refracted by discontinuities in the sub surface. The discontinuities are formed by interfaces between layers having different elastic properties and are called seismic reflectors. The returned energy is recorded by seismic sensors at the sea bottom or near the sea surface. In marine seismic exploration two main methods are used to record the returning seismic energy. One is by using so called hydrophone cables that are towed behind a vessel. This method only records the pressure waves (P-waves) since the shear waves (S-waves) do not propagate through the water column. The other method is to deploy the seismic sensors at the sea bottom (geophones and hydrophones). By doing so both P-waves and S-waves can be recorded and hence more useful data will be recorded and subsequently processed and used for mapping the sub surface.
During the recent years, there has been an increasing activity in improving the results of marine seismic investigations by collecting seismic signals at the seabed instead of, or as a supplement to, the more usual hydrophone streamer signal acquisition.
We will in the following describe the existing, known methods for acquisition of marine seismic data using sensors located at the sea bottom, so called ocean bottom seismic (OBS).
There are basically two main different OBS techniques that are used at present.
The first technique is to deploy an ocean bottom cable with integrated seismic sensors and electrical and/or optical wiring from the sensors to the sea surface where the seismic data is recorded. The seismic energy is generated by a seismic source deployed and towed by a separate vessel called the source vessel. The seismic cable is normally attached during data recording to the cable deploying vessel or another vessel. Real time recording of all sensors takes place onboard the surface vessel. A typical construction of the cable connecting the different sensors that are spaced along the cable (typically with either 25 or 50 m spacing) consists of electrical wires at the center of the cable with a steel wire armor as an outer skin that function as a stress member. The steel wire armor also protects the cable from tearing during the deployment and recovery. This type of cable is sensitive to water leakage through its multiple electrical terminations. Hence, this method has the weakness of being inherently slow since during the deployment and retrieval one has to take into account that the cable is sensitive to any stretch or bending forces. If the cable suffers from leakage, the cable typically has to be retrieved, repaired and redeployed before the data acquisition can commence. The same applies if the cable breaks. Data acquisition using this type of ocean bottom cables are relatively costly because of the slow cable handling and since the common practice is to use three vessels, one source vessel, one cable laying vessel and one combined cable laying and recording vessel.
In the last couple of years a slightly different approach has been in use whereby the recording vessel has been replaced with a recording buoy that also provides the cable with electrical power generated from either a diesel generator or from batteries located in the buoy. All or part of the recorded data is then transmitted via a radio link from the buoy to either the source vessel or the cable vessel. The second present method that is used is to plant and recover autonomous seismic recording nodes to and from the sea bed using a ROV or by simply to drop the recording nodes overboard and then let them slowly descend to the sea bed. In the latter case the seismic recording nodes are recovered to the surface vessel by transmitting an acoustic signal that trigger a mechanism in each node that activates its floating device or releases the node from an anchorage weight such that the node can slowly float up to the sea surface by itself. Both these methods are very time consuming and hence expensive. These types of recording nodes are typically large and heavy.
Another way of using nodes, which has been applied, is to attach the individual nodes to a flexible rope, drop the nodes with slack in the rope between them and then let them descend to the seabed. After the recording is completed the nodes are recovered by winching up the rope.
U.S. Pat. No. 6,024,344 discloses a method for recording seismic data in deep water whereby a free end of a continuous wire is lowered into the water and seismic recorders are subsequently attached to the wire at selected intervals and thereafter lowered to the sea bottom. The wire can also provide electrical communication for power or signals between adjacent recorders or up to a surface vessel.
U.S. Pat. No. 6,657,921 B1 discloses a system for collecting data from underlying geologic formations whereby housings with a first end having a hydrodynamic shaped profile are deployed in the water and then descending fast to the sea bottom. The housings are reconfigured by a controller when coming in contact with the sea bottom. Each housing can contain a marine seismic sensor that can be disconnected from its housing in order to facilitate retrieval of the seismic sensor from the sea bottom.
The need for ROV for most node system operations makes the node handling less efficient and costly. Node surveys are therefore typically coarsely sampled in the receiver domain compared to OBS cable surveys. The cost/sampling issues limit the application of nodes to areas where OBS cable surveys are not an option for operational reasons, for example in the vicinity of infra-structures or in deep water.
The cable based methods described above have typically an interval of 25 m between sensors and allow for a much denser sampling of the underground in the inline direction and in less time compared to methods using separate nodes. However, the cable based systems have limitations when used in deeper waters due to high stress on the cable with its electrical and/or optical wiring as well as an increased probability for water leakage at the electrical terminations between the cable and each sensor house. It has proven to be difficult to operate large receiver spreads due to the operational difficulties mentioned above. As a result, much time is spent on shooting overlaps. In deep waters, cable based systems suffer from an increase in mechanical wear and tear and technical down time to be able to compete with node based systems.
The acquisition methods described above are not viable solutions for larger surveys. Despite their ability to provide better azimuth and offset coverage as well as S-wave data, the efficiency of these systems are too low compared to surface towed streamer seismic systems.